Conventional self-driven synchronous rectifiers are not suitable for use in power converters designed to accommodate a relatively wide input voltage range (e.g., when the maximum input voltage is more than about three times the minimum input voltage). This is because the clamping voltage for the synchronous rectifier, which is derived from the transformer and designed to be barely sufficient when the input voltage is at the minimum, becomes exceedingly large when the input voltage lies in the upper part of the range. The resulting power dissipation and heat generation in the synchronous rectifier drive circuitry makes the power converter inefficient, difficult to cool and subject to failure.
One way to avoid the problem is avoid deriving the clamping voltage from the transformer. An alternative technique involves transferring a control signal from the primary side of the converter to the secondary side. Unfortunately, the control signal must not only be isolated, e.g., by a digital isolation circuit, but also must be delayed, e.g., with an RC delay circuit. The above-described efficiency, thermal and reliability issues are avoided, but the required control circuit is complex, making its layout and manufacturing cost impractical for many practical applications.